With increasing demands for reducing the weight or enhancing the output of automobiles or the like, springs, such as valve springs, clutch springs, and suspension springs, which are used in the engine, clutch, suspension, etc., tend to be higher strength and thinner diameters. Together with this, the properties required for springs, including the resistance to hydrogen embrittlement, the fatigue resistance, and the setting resistance, are becoming increasingly higher. It is strongly desired to provide a spring steel that can manufacture a spring excellent in these properties.
To produce lightweight springs that are excellent in the spring properties, such as the resistance to hydrogen embrittlement and the fatigue resistance, pipe-shaped hollow steels with no weld bead, i.e., seamless pipes are used as material for a spring steel, in place of solid steels, such as a steel bar, which have been used before. The seamless pipe is also called a seamless steel tube.
However, when using the seamless pipe as the material for hollow springs, various problems occur, especially, in terms of manufacturing seamless pipes. That is, to ensure the fatigue strength of the solid steel for use as the material for springs, which are not hollow, generally, a surface layer part of the steel is hardened by shot-peening or the like, thereby applying residual stress to its outer surface. In contrast, the seamless pipe can have its outer peripheral surface subjected to shot-peening in the same way, but its inner peripheral surface cannot undergo the shot-peening. When decarburization occurs at a pipe surface layer located on the inner peripheral surface side of the pipe, adequate hardening on the inner peripheral surface side cannot be obtained during quenching in a spring production procedure, failing to ensure fatigue strength required by springs. Furthermore, the presence of a defect at the surface layer of the inner peripheral surface becomes a stress concentration part, which might cause the breakage of the pipe at an early stage.
During steel production, a small amount of hydrogen, which would cause cracking, is inevitably introduced into and present in the steel. Such a small amount of hydrogen is not problematic for the solid spring, but significantly affects the durability of a hollow spring. In particular, the hollow spring cannot have its inner surface subjected to shot-peening as mentioned above, and thus the hollow spring is required to have an even higher quality of resistance to hydrogen embrittlement than the solid spring.
For these problems, some technical studies have taken place in terms of production of a seamless pipe as a material. In a technique mentioned in Patent Document 1, hot isostatic pressing extrusion is performed on a workpiece of steel to form a hollow seamless pipe shape, followed by spheroidizing annealing, and subsequently extending (drawing) the shape by cold pilger mill rolling, cold drawing, or the like. As a result, according to a seamless steel tube of Patent Document 1, the depth of continuous defects formed at the inner and outer peripheral surfaces of the steel tube can be reduced to 50 μm or less from each surface.
In a technique mentioned in Patent Document 2, a steel bar is hot-rolled, followed by perforation with a gun drill, and then is subjected to cold working (drawn, or rolled). As a result, a hollow seamless pipe for a high-strength spring of Patent Document 2 is produced that can control a C content at the inner and outer peripheral surfaces to 0.10% or more, while reducing the thickness of an entire decarburized layer to 200 μm or less at each of the inner and outer peripheral surfaces.
Patent Document 3 has studied the relationship between the metal microstructure and durability of seamless pipes and thereby disclosing a seamless steel tube for a high-strength hollow spring in which a carbide has a circle-equivalent diameter of 1.00 μm or less.